MB-9217: added node count to bucket-wide statistics screen

Change-Id: I0b7dcdcbfd1c629ce7f830cf0a3c12db8352018d
Reviewed-on: http://review.couchbase.org/29368
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>